Adding Contribution Guidelines to Tornado Web Server

[nbmason14]

Per the issue listed here: #2668 I have taken a first pass at Contribution Guidelines for Tornado Web Server

If I am missing key information or need to make any edits please let me know and I will fix them

[bdarnell]

This seems fine, but it's fairly generic. If this is all CONTRIBUTING.md is going to say, I'd rather just link to some generic contribute-to-open-source documentation. But instead, we should probably add some tornado-specific details:

• We use black as a code formatter, and enforce this in CI.
• We use type annotations and validate them with mypy.
• Most pull requests should include new or updated tests to make sure the change works.
• tox is the simplest way to run the tests locally. tox -e py37-full is recommended most of the time instead of running every configuration. tox -e py3-mypy,py3-lint checks formatting and type annotations.
• We use sphinx's autodoc extension to extract docs from docstrings.
• pip install -r maint/requirements.txt can be used to install the specific versions of tools like tox, sphinx, and linters used by Tornado developers.